Turbine engines, such as single shaft industrial gas turbines, are designed to operate at a constant design turbine inlet temperature under any ambient air temperature (i.e., the compressor inlet temperature). This design turbine inlet temperature allows the engine to produce maximum possible power, known as base load. Any reduction from the maximum possible base load power is referred to as part load operation. In other words, part load entails all engine operation from 0% to 99.9% of base load power.
Part load operation may result in the production of high levels of carbon monoxide (CO) during combustion. One known method for reducing part load CO emissions is to bring the combustor exit temperature or the turbine inlet temperature near that of the base load design temperature. It should be noted that, for purposes of this disclosure, the terms combustor exit temperature and turbine inlet temperature are used interchangeably. In actuality, there can be from about 30 to about 80 degrees Fahrenheit difference between these two temperatures due to, among other things, cooling and leakage effects occurring at the transition/turbine junction. However, with respect to aspects of the present invention, this temperature difference is insubstantial.
To bring the combustor exit temperature closer to the base load design temperature, mass flow of air through a turbine engine 10 (FIG. 5) can be restricted by closing the compressor inlet guide vanes (IGV) (not shown), which act as a throttle at the inlet of the compressor 12. When the IGVs are closed, the trailing edges of the vanes rotate closer to the surface of an adjacent vane, thereby effectively reducing the available throat area. Reducing the throat area reduces the flow of air which the first row of rotating blades can draw into the compressor. Lower flow to the compressor leads to a lower compressor pressure ratio being established in the turbine section 14 of the engine 10. Consequently, the compressor exit temperature decreases because the compressor 12 does not input as much energy into the incoming air. Also, the mass flow of air through the turbine 10 decreases, and the combustor exit temperature increases.
Typically, some of the compressor exit air 16 from the combustor shell 18 is used as cooling air supplied directly to structure 20 of the turbine 10 adjacent to the first row of blades 22a. This structure can include the outer casing, blade rings, and ring segments. In addition, some compressed air, comprising compressor bleed air, may be piped directly out of the compressor through piping 24b, 24c, 24d. This compressor bleed air is directed to structure, e.g., stationary vane structure 26b, 26c, 26d, near second, third and fourth row blades 22b, 22c, 22d, within respective second, third and fourth stages 28b, 28c, 28d of the turbine section 14.
CO increases rapidly as gas turbine engine load is reduced below approximately 60%. Once the IGVs have been closed to their limit, and the engine's exhaust temperature limit has been reached, then power typically may be reduced only by decreasing turbine inlet temperature (TIT). TIT reduction corresponds to a decrease in the combustion system's primary zone temperature (T_PZ), resulting in CO and unburned hydrocarbons (UHC) being produced due to quenching of the combustion reactions in the turbine hot gas path. To prevent CO from increasing as engine load decreases, the T_PZ must be maintained at a high level.